I completely get what you mean @vegabook. I too struggled longer than I’d like to admit before I got my assymetric RSA encryption working. Please see below for a module I wrote, that hopefully explains things well enough.

defmodule Ls.Crypto.Assymetric do
  @moduledoc """
  Functionality releated to assymetric encryption, e.g. public key encryption.

  Create the keys used here in the following way:

  # Public key.
  openssl genrsa -out private.pem 2048

  # Private key.
  openssl rsa -in private.pem -pubout -out public.pem
  """
  @encrypt_decrypt_opts [rsa_padding: :rsa_pkcs1_oaep_padding]

  def encrypt_rsa(text, key_path) do
    key = load_key(key_path, :RSAPublicKey)
    :public_key.encrypt_public(text, key, @encrypt_decrypt_opts)
  end

  def decrypt_rsa(cipher_text, key_path) do
    key = load_key(key_path, :RSAPrivateKey)
    :public_key.decrypt_private(cipher_text, key, @encrypt_decrypt_opts)
  end

  defp load_key(key_path, expected_key_type) do
    {:ok, raw_key} = File.read(key_path)

    [enc_key] = :public_key.pem_decode(raw_key)
    pem_entry = :public_key.pem_entry_decode(enc_key)

    # Check that we loaded a key that's of the expected type (private vs public).
    # It's an easy mistake to load the wrong key, so this check can
    # save a lot of debugging time.
    if elem(pem_entry, 0) != expected_key_type do
        raise("Expected a key of type #{expected_key_type}, got #{elem(pem_entry, 0)}")
    end      

    pem_entry
  end
end

Caveats

1. A .pem file is a container format, so it can contain many different things. This is why we need to do the :public_key.pem_decode/1 and :public_key.pem_entry_decode/1. My code assumes the PEM file contains just a single key.

2. It works on my machine and for my particular use case. It’s definitely not the most general, universally applicable way of doing things .

It’s a thing of beauty!

image1200×766 133 KB

I had the exact “fun” you’re talking about. Crypto is really interesting, deep and complex. Learning more about how any part of it works is well worth the effort (or rabbit hole). Good luck!